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                [Usaco2017 Open]COWBASIC                </h1>
                <p>时间限制：10s&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;  空间限制：128MB</p>			</div>
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                                <h3>题目描述</h3><p><div>Bessie has invented a new programming language, but since there is no compiler yet, she needs your h</div>
<div>elp to actually run her programs.COWBASIC is a simple, elegant language. It has two key features: ad</div>
<div>dition and MOO loops. Bessie has devised a clever solution to overflow: all addition is done modulo&nbsp;</div>
<div>109+7. But Bessie's real achievement is the MOO loop, which runs a block of code a fixed number of t</div>
<div>imes. MOO loops and addition can, of course, be nested.Given a COWBASIC program, please help Bessie&nbsp;</div>
<div>determine what number it returns.</div>
<div></div>
<p></p></p><hr/><h3>输入格式</h3><p><div>You are given a COWBASIC program at most 100 lines long, with each line being at most 350 characters</div>
<div>&nbsp;long. A COWBASIC program is a list of statements.</div>
<div></div>
<div>There are three types of statements:</div>
<div></div>
<div>&lt;variable&gt; = &lt;expression&gt;</div>
<div></div>
<div>&lt;literal&gt; MOO {</div>
<div>&nbsp; &lt;list of statements&gt;</div>
<div>}</div>
<div></div>
<div>RETURN &lt;variable&gt;</div>
<div></div>
<div>There are three types of expressions:</div>
<div></div>
<div>&lt;literal&gt;</div>
<div></div>
<div>&lt;variable&gt;</div>
<div></div>
<div>( &lt;expression&gt; ) + ( &lt;expression&gt; )</div>
<div></div>
<div>A literal is a positive integer at most 100,000.</div>
<div></div>
<div>A variable is a string of at most 10 lowercase English letters.</div>
<div></div>
<div>It is guaranteed that no variable will be used or RETURNed before it is defined. It is guaranteed th</div>
<div>at RETURN will happen exactly once, on the last line of the program.</div>
<div></div>
<p></p></p><hr/><h3>输出格式</h3><p><div>
<div>Output a single positive integer, giving the value of the RETURNed variable.</div>
<div></div>
<div>Scoring</div>
<div></div>
<div>In 20 percent of all test cases - MOO loops are not nested.</div>
<div>In another 20 percent of all test cases - The program only has 1 variable. MOO loops can be nested.</div>
<div>In the remaining test cases, there are no further restrictions.&nbsp;</div>
<div></div>
</div>
<div></div>
<p></p></p><hr/><h3>样例输入</h3><pre>x = 1
10 MOO {
  x = ( x ) + ( x )
}
RETURN x

SAMPLE OUTPUT:

1024

This COWBASIC program computes 210.
SAMPLE INPUT:

n = 1
nsq = 1
100000 MOO {
  100000 MOO {
    nsq = ( nsq ) + ( ( n ) + ( ( n ) + ( 1 ) ) )
    n = ( n ) + ( 1 )
  }
}
RETURN nsq

</pre><hr/><h3>样例输出</h3><pre>4761
This COWBASIC program computes (10^5+10^5+1)^2 (modulo 10^9+7). </pre><hr/><h3>提示</h3><p>没有写明提示</p><hr/><h3>题目来源</h3><p>Platinum</p>
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